Many high-speed integrated devices, such as a synchronous memory device 40 shown in FIG. 1, perform operations in a predetermined sequence. These operations are generally performed responsive to respective command signals issued by a command generator, such as a memory controller 44.
It will be understood by one skilled in the art that the block diagram of FIG. 1 omits some signals applied to the memory device 40 for purposes of brevity. Also, one skilled in the art will understand that the command signals COM may be composed of a combination of other signals or may be a packet of control data. In either case, the combination of signals or packet is commonly referred to as simply a command. The exact nature of these signals or packet will depend on the nature of the memory device 40, but the principles explained above are applicable to many types of memory devices, including synchronous DRAMs and packetized DRAMs. Also, although the timing control by issuing command signals according to a fixed relationship with the clock signal will be explained with reference to memory devices, the principles described herein are applicable to other integrated circuits that utilize counters or related switching signals responsive to a clock signal.
Timing of operations within the device 40 is determined by a logic control circuit 42 controlled by an internal clock signal CKBUF. In a synchronous DRAM, the logic control circuit 42 may be realized conventionally. In a packetized memory system, the logic control circuit may include command sequencing and decoding circuitry.
Timing of signals outside of the memory device 40 is determined by an external clock signal CKIN that is produced by an external device 44 such as a memory controller. Usually, operations within the memory device 40 must be synchronized to operations outside of the memory device 40. For example, commands and data are transferred into or out of the memory device 40 on command and data busses 48, 49, respectively, by clocking command and data latches 50, 52 according to the internal clock signal CKBUF. Command timing on the command bus 48 and data timing on the data bus 49 are controlled by the external clock signal CKIN. To transfer commands and data to and from the busses 48, 49 at the proper times relative to the external clock signal CKIN, the internal clock signal CKBUF must be synchronized to the external clock signal CKIN.
To ensure that the clock signals CKBUF, CKIN can be synchronized, the internal clock signal CKBUF is derived from the external clock signal CKIN. A buffer amplifier 46 buffers the external clock signal CKIN to produce a buffered version of the external clock signal CKIN as the internal clock signal CKBUF. The buffer amplifier 46 is a conventional differential amplifier that provides sufficient gain and appropriate level shifting so that the buffered clock signal CKBUF can drive circuits within the memory device 40 at CMOS levels.
The buffer amplifier 46 also induces some time delay so that the buffered clock signal CKBUF is phase-shifted relative the external clock signal CKIN. As long as the phase-shift is very minimal timing within the memory device 40 can be synchronized easily to the external timing.
Unfortunately, as the frequency of operation of the memory device 40 increases, the time delay induced by the buffer amplifier 46 may become significant. Consequently, commands or data supplied by the memory controller 44 may be gone from the command or data bus 48, 49 before the latches 50, 52 are activated on the appropriate edge of the buffered clock signal CKBUF. To prevent the latches 50, 52 from missing commands that arrive synchronously with the external clock CKIN, the memory device 40 may be operated at lower frequencies. However, lower frequency operation of memory devices typically reduces the speed of operation undesirably.
To improve synchronization of the internal and external timing, a prior art memory device 60 shown in FIG. 2 includes an analog delay-locked loop 62 that receives the buffered clock signal CKBUF and produces a synchronized clock signal CKSYNC that is synchronized to the external clock signal CKIN. To compensate for the delay of the buffer amplifier 46, the synchronized clock signal CKSYNC is phase-shifted relative to the buffered clock signal CKBUF by an amount offsetting the delay of the buffer amplifier 46. Because the synchronized clock signal CKSYNC is synchronized and substantially in phase with the external clock signal CKIN, commands and data arriving on the command bus 48 or data bus 49 can be synchronized to the external clock CKIN through the synchronous clock signal CKSYNC.
One problem with the memory device 60 of FIG. 2 is that conventional delay-locked loops 62 typically operate only over a narrow frequency band. Consequently, the memory device 60 may not operate properly in multifrequency environments or in a wide range of applications.
Moreover, many conventional analog delay-locked loops include relatively sophisticated analog components that are not always easily integrated with digital memory components. Also, as operating conditions vary, the delay of the buffer amplifier 46 can vary, thereby causing corresponding variations in the phase shift. If the delay-locked loop 62 does not adjust the phase shift of the synchronous clock signal CKSYNC accordingly, operations within the device 40 may not remain properly synchronized to the external clock CKIN.